Various more or less fast and accurate methods have been proposed to measure the resistivity index of rocks. Several combinations of these techniques have been proposed. A known method for measuring the resistivity index essentially consists in combining a multicore air-water "semipermeable membrane" desaturation technique with a two-electrode resistivity measuring technique and in calculating the average saturation by weight difference. It appears that this method is very imprecise and depends too much on the handling quality; it is very slow and does not take account of a possible wettability effect.
Different solutions using the same principle have been found to improve the precision thereof with, in return, greater implementation complexity and a correlative cost increase. It is for example possible to
carry out separate measurements on individual core samples in order to better control the capillary equilibrium and thus to obtain uniform saturation profiles, PA1 implement a technique using four electrodes so as to avoid overestimation of the electric resistance, PA1 implement a continuous injection technique in order to accelerate experiments when only curve Ir is necessary, as described for example by de Waal et al., 1991, PA1 optimize the duration of the desaturation process by using a micropore membrane and by reducing the length of the core samples as described by PA1 Longeron D. et al., "Water-Oil Capillarity Pressure and Wettability Measurements Using Micropore Membrane Technique" SPE 30006, 1995, or PA1 Fleury M. et al., "Combined Resistivity and Capillarity Pressure Measurements using Micropore Membrane Technique" Proceeding of the International Symposium of the Society of Core Analysts, Montpellier 1996. PA1 Jing et al., "Resistivity Index from non Equilibrium Measurements using Detailed in situ Saturation Monitoring", SPE 26798 Offshore European Conference 1993. PA1 it uses electrodes whose longitudinal extension is relatively great in relation to the length of the sample but shorter than this length, so selected as to involve the largest possible part of the volume of the sample in the impedance measurements while avoiding short circuits through the ends of the sample, likely to distort measurements, and PA1 at least one injection pressure stage is applied for the second fluid and precise continuous measurement of the complex electric impedance variations of the sample is carried out at several frequencies during a phase of displacement of the saturating fluid (drainage phase or imbibition phase), measurement being achieved without waiting for a capillary pressure equilibrium to be reached in the sample in response to each pressure stage. PA1 to draw a very precise drainage continuous resistivity index curve in a short time (about 2 days for a typical 100 mD sandstone whereas the typical time required with the continuous injection technique is often of the order of two weeks), PA1 the method is not linked with a capillary pressure equilibrium, PA1 the effect of non -uniform saturation profiles during measurement is negligible. This is due to the combination of three factors: (i) the radial resistivity measuring technique, (ii) the presence of semipermeable filters on the outlet side, (iii) all the volume of the core is analysed by means of electric measurements (this is verified when the diameter of the core is greater than its length).
However, when the measurement of Ir is linked with the determination of the capillary pressure curves, it is very difficult to reduce the duration of the experiments.
Other desaturation techniques such as the centrifugal method can be selected and implemented in the same way as the aforementioned "semipermeable membrane" method with multiple cores. Implementation of the centrifugal method also proved to be imprecise because of the accumulation of two important problems linked with the saturation profile and the contact resistance. On account of the known Archie relation connecting Ir and Sw (Ir=Sw.sup.-n), the measurements are very sensitive to saturation. It is then also possible to determine the saturation profile (during a fluid injection for example) and to reduce the duration of the experiment by using a device for measuring the local saturation in situ and multiple electrodes as described by
The method according to the invention allows continuous measurement of the resistivity index of a porous solid sample, combining rapidity, precision and low cost, while avoiding the drawbacks of the prior methods and notably the obligation to perform in-situ saturation monitoring, which is long and expensive.